The present invention concerns a deposited multi-layer device having a plastic substrate with an inorganic thin film layer on the surface and being suitable, for instance, to a flat panel display used for office automation equipment, a television apparatus and the like.
In a terminal unit for office automation equipment and a liquid crystal television apparatus, the use of a large area liquid crystal panel has been strongly requested and an active matrix system in which a switch is placed on each picture element to maintain a necessary voltage is disclosed in Japanese Patent Application Laid-Open (KOKAI), Nos. 260,219 (1986) and 62,333 (1987).
On the other hand, extensive studies were made recently for reducing the weight and cost of liquid crystal panels and Japanese Patent Application Laid-Open (KOKAI), No. 47,769 (1989), for instance, discloses a switching device with a plastic substrate.
However, when a deposited multi-layer switching device, for instance, is formed on a plastic substrate, peeling of the deposited multi-layer is caused by a deformation or a curl of the substrate. Further, since a process for fabricating the device includes a photolithographic step, in which a dipping of the device together with the substrate in an aqueous solution of acid or alkali or water is necessary, an acid, an alkaline or water remains in the substrate and can be a cause of spoiling the device later. Further, when a fine pattern is to be formed in the device, shift of a part of the pattern is caused due to an elongation and a shrinkage of the substrate and results in a problem, for instance, that a simultaneous exposure of a large area pattern is difficult or an anisotropy in an elongation and shrinkage of the substrate makes the patterning difficult.
On the other hand, in the fabrication of a liquid crystal display device with a plastic substrate, alignment of liquid crystal molecules requires a specific and complicated treatment on the substrate. In order to avoid this, Japanese Patent Publication, No. 47,769 (1989) discloses that when an SiO.sub.2 layer is formed on one surface of the substrate, the aligning can be applied in the same manner as on a glass substrate. However, when an SiO.sub.2 thin film is coated on a plastic substrate and a deposited multi-layer device is fabricated thereon, crack formations on the SiO.sub.2 thin film are often observed and give an insufficient reliability to the device.
In order to improve crack resistance and humidity resistance of the SiO.sub.2 film, a technique of applying a plasma treatment with a fluorine containing gas to a surface of the film has been disclosed in Japanese Patent Application Laid-Open (KOKAI), No. 25,543 (1989). However, since this method is merely a surface treatment of the film with fluorine and not the fluorination of the entire film and further since the fabrication of the SiO.sub.2 film is based on a wet process by a spin-on-glass (hereinafter referred to as "SOG") method, shrinkage of the film is inevitable and still is a cause of cracks.
The present inventors have made an earnest study with an aim of fabricating a reliable and inexpensive deposited multi-layer device with a plastic substrate and, as a result, have realized that curl is the largest problem of the deformation of a plastic substrate upon fabrication of the device, and that causes of the deformation are an internal stress in deposited thin films, a relation between the above stress and the stress of the inorganic substance thin film layer, thermal expansion and shrinkage of the plastic substrate and further, more or less, a swelling of the plastic material of the substrate. It has been found that forming an inorganic substance thin film on the substrate is an effective countermeasure to prevent these causes of deformation. Based on these findings, they have attained the present invention. Further, when the thickness of the inorganic substance thin film is increased in order to reduce the generation of pinholes and to increase the rigidity of the film, cracks on the film are often observed and it has been confirmed that the use of fluorine-containing silicon compound as the thin film is effective for preventing the cracking.